Aromatic hydrocarbons, such as benzene, toluene, xylene, etc., are useful as fuels, solvents, and as feeds for various chemical processes. Of the xylenes, para-xylene (“p-xylene”) is particularly useful for manufacturing phthalic acids such as terephthalic acid, which is an intermediate in the manufacture of synthetic fibers such as polyester fibers. Xylenes can be produced from naphtha, e.g., by catalytic reforming, with the reformate product containing a mixture of xylene isomers and ethylbenzene. Separating the xylenes from the mixture generally requires stringent separations, e.g., separations utilizing superfractionation and multistage refrigeration steps. Such separations are characterized by complexity, high energy-usage, and high cost. Processes having a relatively large p-xylene yield are desired because they would lessen the need for such stringent separations.
One method for increasing p-xylene yield involves alkylating toluene with methanol over a solid acid catalyst. See, e.g., Yashima et al., in the Journal of Catalysis 16, pp. 273-280 (1970), which discloses selectively producing p-xylene over a temperature in the range of 200° C. to 275° C., with the maximum yield of p-xylene occurring at 225° C.
More recently, U.S. Pat. No. 6,504,072 discloses a method for improving p-xylene yield by alkylating toluene with methanol using a catalyst comprising severely-steamed ZSM-5 and an oxide modifier.
There is a continuing need to provide processes which are highly selective for the production of p-xylene, particularly processes utilizing relatively low-value hydrocarbon feed.